The invention relates to a metal-semiconductor-field effect transistor (MESFET).
Field effect transistors are known in which the gate electrode is supported directly on the semiconductor surface between the source area and the drain area and forms a metal-semi-conductor (Schottky) contact. Such field effect transistors are designated as MESFET's. Such a MESFET is built up, for example, on a semiconductor layer located on an insulating substrate, whereby the layer thickness generally amounts from 0.1 to 0.5 .mu.m and the doping of this semiconductor material has a magnitude of about 10.sup.17 particles per cm.sup.3. The contact areas for source and drain generally have a distance from one another of about 3 to 7 .mu.m and a strip-shaped Schottky-contact area with a width of 0.5 to 2 .mu.m lies between these two areas. Gallium arsenide is preferably considered as the semiconductor material for MESFET's. All the three areas have the same conductivity type.
For power MESFET's, the parasitic resistances at the contacts and in the semiconductor layer are to be kept small and a large voltage between source and drain area can be applied without an electrical puncture or breakthrough occurring in the semiconductor layer between these areas.
Up to now, this problem was solved in such manner that one provided a relatively thick semiconductor layer and applied the contact strip for the gate in a somewhat recessed area of the semiconductor layer. It was thereby achieved that the semiconductor layer has a small thickness below the contact strip serving as the gate required for the function of a MESFET. The thicker layer parts beyond the gate area contribute to a reduction of the parasitic resistances. It has likewise been suggested to provide both the source area as well as the drain area with an increased doping of, for example, 10.sup.18 particles per cm.sup.3. Accordingly, the contact resistance between the material of the semiconductor layer and the contact metallization located thereon was reduced by the ohmic junction, the path resistance was reduced in the semiconductor layer, and higher voltages could be applied. Moreover, by so doing, the noise property could also be improved. Concerning this state of the art, the publications IEEE MT-24, 1976, 312-317; Proceedings of the 6th International Symposium concerning Gallium arsenide and Related Compounds, St. Louis (1976), Pages 262-270; and IEEE ED-24 (1977), 1129-1130 all incorporated herein by reference, are pertinent.
According to the IEEE ISSCC Digest of Technical Papers (1978), 118-119, incorporated by reference, good power MESFET's can also be achieved when the doping in the source area and in the drain area lies at only about 10.sup.17 particles per cm.sup.3 and the thickness of the semiconductor layer is selected correspondingly large.